Rdeče zapolnitve megalodontidnih školjk iz Doline pod Peski v Krnskem pogorju

Slapnik, Lucija (author), Rožič, Boštjan (mentor)

V sklopu diplomske naloge smo preučili rdeče zapolnitve megalodontidnih školjk iz norijsko-retijskega Dachsteinskega apnenca v dolini pod Peski v Krnskem pogorju v Julijskih Alpah. Praviloma se megalodontidne školjke pojavljajo v podplimskem faciesu in so zapolnjene oziroma neomorfno nadomeščene z belkastim kalcitnim cementom. V našem primeru se poleg klasičnih zapolnitev pojavljajo tudi takšne, kjer so megalodontide popolnoma ali vsaj deloma zapolnjene z rdečim ali zelenim, rahlo lapornatim apnencem. Najlepše so tovrstne zapolnitve, razgaljene na plastnici znotraj zgornjetriasnega Dachsteinskega apnenca v dolini pod Peski v Krnskem pogorju. Poleg zapolnitev megalodontidnih školjk smo preučili tudi bližnje neptunske dajke, ki so zapolnjeni z apnenčevo brečo, z rdečkastim in ponekod rumeno-zelenim apnenčastim vezivom. Na raziskovanem območju se pojavljata dve smeri neptunskih dajkov z nekoliko različnimi zapolnitvami. Hipoteza diplomske naloge je bila, da je rdečkast sediment v megalodontidah enak vezivu breč v neptunskih dajkih. Na lokaciji smo odvzeli vzorce megalodontidnih školjk z rdečimi apnenčastimi zapolnitvami in belim kalcitnim cementom ter vzorce iz poleg prisotnih neptunskih dajkov. Nato smo pripravili 43 zbruskov, ki smo jih fotografirali, skenirali ter preučili z optičnim polarizacijskim mikroskopom. Z optično mikroskopijo smo ugotovili štiri generacije zapolnitev školjk ter dokazali, da je vsaj del dajkov zapolnjen z zgornjekrednim apnencem. Starosti sedimenta v megalodontidah žal nismo mogli dokazati, saj v sedimentu ni bilo biostratigrafsko značilnih fosilov, vendar domnevamo, da je sediment iz obdobja jure, ko je ekstenzijska tektonika povzročila razpad in poglabljanje Julijske karbonatne platforme ter posledično formiranje neptunskih dajkov. Sediment, naj bi se iz območja dajkov potem lateralno izpiral v delno raztopljene lupinice školjk in polžev v matični kamnini. Za nadaljnje raziskave bi bilo priporočeno izvesti še katodoluminiscenčno raziskavo, s katero bi še natančneje ločili generacije cementov.

• Language: Slovenian
• Keywords: Južne Alpe, Krnski pokrov, Dachsteinski apnenec, zgornji trias, megalodontidne školjke, neptunski dajki, sedimentologija.
• Work type: Bachelor thesis/paper
• Organization: NTF - Faculty of Natural Sciences and Engineering
• Year: 2018
• PID: 20.500.12556/RUL-103683
• COBISS.SI-ID: 1418846
• Publication date in RUL: 21.09.2018

Secondary language

• Language: English
• Title: Red Infillings of Megalodontid Bivalves from Pod Peski Valley in the Krn Mountain Range

Abstract

The thesis studies the reddish infilling of megalodontid bivalves of the Norian-Rhaetian Dachstein Limestone in the valley below the Peski Mountain in the Krn Range of the Julian Alps. Megalodontid shells are usually found in subtidal facies and are filled or neomorphically replaced with white calcite cement. In our case, in addition to the classic fillings, there are also specimens completely or at least partially filled with red or green, slightly marly limestone. A layer inside the Dachstein Limestone in the valley below the Peski Mountain provides the best insight into these picturesque infillings. In addition to the filling of the megalodontid bivalves, we also examined the nearby neptunian dikes, which are filled with limestone breccia with reddish and sometimes yellow-green limestone matrix. There are two generations of neptunian dikes with different shivering and fillings in the explored area. The research hypothesis of the thesis was that the reddish sediment in the megalodontides, and of the matrix in the breccias of the neptunian dikes was the same. Samples of megalodontid bivalvia with red limestone fillings, white calcite cement, as well as samples from the neptunian dikes were collected. Fourty-three thin-sections were photographed, scanned and examined under a optical polarizing microscope. Using optical microscopy, four generations of fillings bivalves were defined, and it has been proven that at least some of the neptunian dikes were filled with Upper Cretaceous limestone. Unfortunately, we could not prove the age of the sediment in megalodontides, since there were no biostratigraphically characteristic fossils in the infill , but we assue it is a Jurassic sediment as in this period the extensional tectonics disintegrated and deepened the Julian carbonate platform, and consequently caused the formation of neptunian dikes. We assume that the matrix from dikes was laterally washed into the partially dissolved shells of bivalves and gastropods in the host rock. Further research should focus on carrying out a cathodoluminescence study to further define the cement generations.

• Keywords: Southern Alps, Krn nappe, Dachstein Limestone, Upper Trias, megalodontid bivalves, neptunian dikes, sedimentology.